One of various possibilities to achieve ply bonding between at least two plies of tissue paper without the use of glue is disclosed in WO-A-99/33646. The known device comprises two rollers forming a nip through which at least two plies which are to be bonded are fed. At least the outer periphery of one of the rollers is entirely covered with abrasive material such as the material used for sandpaper so as to achieve an irregular rough surface. This abrasive material is pressed into the nipped plies, whereby ply bonding is achieved.
However, an irregular rough surface structure is imprinted into at least one of the plies over the entire surface. Therefore, the outer appearance of the combined plies is irregular or the ply bonding is (almost) not visible. In addition, it will not be possible to create volume between the plies by embossing and even pre-embossed webs with a defined thickness would be flattened by compressing of the ply.
To enhance the visual appearance of the bonded plies, WO-A-99/33646 additionally suggests a subsequent embossing step. The subsequent embossing requires additional devices with the associated additional steps. This, in turn, increases the complexity of the apparatus and, hence, the manufacturing costs of the final product.
Furthermore, EP-A-1 216 818 discloses an apparatus for bonding at least two plies of a fibrous web comprising a first roller having an outer periphery, a plurality of embossing protuberances being provided on the outer periphery and a second roller having an outer periphery and consisting of rubber at least in the radial direction, the second roller together with the first roller forming a nip through which the at least two plies are to be fed. To achieve ply bonding spikes are either located on top of the embossing protuberances of the first roller or a plurality of spikes are provided on the outer periphery of the second roller. The spikes, however, have the disadvantage that the fibrous web in the area of the spikes is perforated leading to a damage of the fiber structure. In addition, such a perforation and damage is considered detrimental from the view point of visual appearance of the final product. Moreover, the tips of the spikes are subjected to substantial wear by the pressure exerted in the nip resulting in high maintenance work and exchange operations. This, in turn, increases the manufacturing costs of the final product.
Handkerchiefs based on tissue paper are being produced by using foot-to-foot embossing techniques, foot-to-flat embossing techniques or so called Union embossing. In order to achieve mechanical ply-bonding the embossing protuberances of different rollers must match with each other. However, such matching is becoming more and more difficult if the width of the paper roll exceeds approximately 1 m.
Mechanical ply-bonding is also possible by using the so called knurling technique. Knurling is normally been carried out by applying steel-to-steel knurling apparatuses. In order to achieve mechanical ply-bonding, it is necessary to use a lot of knurling stations along the width of the paper roll (e.g. usually up to 26 knurling stations for producing bathroom tissue paper having a width of 2.70 m), thereby increasing the maintenance costs of the ply-bonding process. The resulting tissue products of such a knurling process are characterized by an optical appearance which does not satisfy all consumers demands.
Ply-bonding can also be achieved by using adhesives according to well-known process (goffra incolla, nested, pin-to-pin). However, such ply-bonding by means of adhesives will result in increased production costs and an increased stiffness of the final product depending on the embossing pattern and the dot density.